Growth hormone-binding protein in human lymph

The high affinity GH-binding protein (GHBP) is a soluble circulating ectodomain of the GH receptor (GHR). In humans, it is thought to be released from the plasma membrane-bound GHR by proteolysis at or near the transmembrane domain. GHBP modulates GH action by 1) intravascular complex formation, and 2) competing with the GHR for ligand in tissues (interstitial complex formation). Little is known about the tissue source(s) of GHBP, the local regulation of GHBP generation, or its concentration in the interstitium. To begin addressing these questions, we studied GHBP levels in peripheral lymph, whose composition approximates that of interstitial fluid. Lymph was collected in 13 healthy adult men from cannulated lymphatic vessels in the calf. Venous and arterial blood samples were collected from the femoral vein and radial artery contemporaneously with lymph collection. Potential GHBP production by endothelial or blood cells was assessed by examining conditioned medium from in vitro cell cultures. GHBP activity was measured by standardized GH binding/column chromatography assay. GHBP was consistently and significantly lower in lymph (mean +/- SD, 4.6 +/- 1.2% GH bound/200 microL) than in venous (14.1 +/- 3.0%) or arterial (14.9 +/- 3.6%) plasma. Conditioned medium from endothelial or blood cell cultures did not contain detectable GHBP. We conclude that the level of GHBP in peripheral lymph is substantially lower than that in the peripheral circulation, and that components of the vasculature are not important sources of GHBP. These findings suggests that 1) the main tissue sources of GHBP in man are the central organs (especially liver); and 2) transcapillary diffusion of GHBP into the interstitial space is restricted.